Update .travis.yml
Update .travis.yml

Test 3.4

Make the pydap tests optional

markelg/xray,shoyer/xray,pydata/xarray,xray/xray,pydata/xarray,markelg/xray,hetland/xray,pydata/xarray,drewokane/xray,markelg/xray,kjordahl/xray,jhamman/xarray,clarkfitzg/xray,rabernat/xray,jcmgray/xarray,shoyer/xarray,cpaulik/xray,jhamman/xray,chunweiyuan/xarray,jhamman/xarray,kjordahl/xray,cpaulik/xray,petercable/xray,NicWayand/xray,hetland/xray,shoyer/xarray,jhamman/xarray,rabernat/xray,clarkfitzg/xray